The iBiquity Digital Corporation HD Radio™ system is designed to permit a smooth evolution from current analog amplitude modulation (AM) and frequency modulation (FM) radio to a fully digital in-band on-channel (IBOC) system. This system delivers digital audio and data services to mobile, portable, and fixed receivers from terrestrial transmitters in the existing medium frequency (MF) and very high frequency (VHF) radio bands. Broadcasters may continue to transmit analog AM and FM signal simultaneously with the new, higher-quality and more robust digital signals, allowing themselves and their listeners to convert from analog to digital radio while maintaining their current frequency allocations.
The system provides a flexible means of transitioning to a digital broadcast system by providing three waveform types: Hybrid, Extended Hybrid, and All Digital. The Hybrid and Extended Hybrid types retain the analog FM signal, while the All Digital type does not. All three waveform types conform to the currently allocated spectral emissions mask. Details on the Hybrid, Extended Hybrid, and All Digital waveforms are shown in United States Patent Application Publication No. 2004/0076188, which is hereby incorporated by reference.
The digital signal is modulated using Orthogonal Frequency Division Multiplexing (OFDM). OFDM is a parallel modulation scheme in which the data stream modulates a large number of orthogonal subcarriers, which are transmitted simultaneously. OFDM is inherently flexible, readily allowing the mapping of logical channels to different groups of subcarriers.
During the transition from analog to digital broadcasting, it is envisioned that the predominant transmit modes for the HD Radio™ system will be the Hybrid modes. The Hybrid signal includes the conventional analog signal (for compatibility with existing radios) as well as digital signal subcarriers carrying the same analog audio content, but in higher-quality digital format. The digital signal is delayed with respect to its analog counterpart such that this time diversity can be used to mitigate the effects of short signal outages. In these modes, hybrid-compatible digital radios will incorporate a feature called “blend” which attempts to smoothly transition from outputting digital audio to analog audio during initial tuning, or whenever the digital waveform quality falls below an acceptable level. The blend function is described in U.S. Pat. Nos. 6,590,944 and 6,735,257, which are hereby incorporated by reference.
Blending will typically occur at the edge of digital coverage and at other locations within the coverage contour where the digital waveform is corrupted. When a short outage does occur, such as traveling under a bridge, the loss of digital audio is replaced by an analog signal. When blending occurs, it is important that the content on the analog audio and digital audio channels are aligned in both time and level to ensure that the transition is barely noticed by the listener. Optimally, the listener will notice little other than possible inherent quality differences in analog and digital audio at these blend points. However, if the broadcast station does not have the analog and digital audio signals aligned, then the result could be a harsh sounding transition between digital and analog audio. The misalignment may occur because of audio processing differences between the analog audio and digital audio paths at the broadcast facility. Furthermore the analog and digital signals are typically generated with two separate signal generation paths before combining for output. The use of different analog processing techniques and different signal generation methods makes the alignment of these two signals nontrivial. The blending must be smooth and continuous, which can happen only if the analog and digital audio is both time and level aligned.
The alignment or calibration of an HD Radio™ broadcast station's digital and analog signals is presently done manually with test equipment located at the transmitter site. This calibration requires the use of a test signal and special measurement equipment used to measure the time and level differences of the analog and digital signals. It also accounts for the intentional diversity delay imposed on the analog signal path. Furthermore the relative delays may change occasionally if the audio processing is changed, which may occur if or when the broadcast changes from music to news, for example. It is presently impractical, or cumbersome, to manually realign the signals when these modifications occur. Therefore it would be a significant benefit and convenience if the ability to automatically detect and correct alignment errors were available.